1. Field of the Invention
The present invention relates generally to an optical filter, and more specifically to a semiconductor optical filter that includes a distributed feedback semiconductor laser structure and is capable of tuning or changing a wavelength of resonance-amplified light, which is caused by a plasma effect that is due to a change in carrier density within the laser structure. The present invention also relates to an optical communication system using such an optical filter.
2. Related Background Art
In conventional optical filtering devices or demultiplexers used for selecting and transmitting an arbitrary optical signal from among wavelength-multiplexed optical signals, there have been proposed various types of devices. Among them, many distributed feedback (DFB) type and distributed Bragg reflector (DBR) type tunable optical filters using gratings have been proposed as a demultiplexer applicable to a high-density wavelength division multiplexing system or a light frequency division multiplexing system, which is expected as a future optical transmission system because of its high wavelength resolution, its capability of tuning a wavelength to be selected, its suitability to integration, etc. Japanese patent laid-open application Nos. 62-2213, 62-241387 and 63-133105 and U.S. Pat. No. 5,084,897 disclose such tunable demultiplexers or optical filter devices.
FIG. 1 shows a schematic cross section of such a prior art tunable optical filter.
In the structure of FIG. 1, a grating 53 is formed at the right side portion of a waveguide layer 52, and an active layer 51 is formed at an upper portion of the left side of the waveguide layer 52. Electrodes for injecting current are respectively provided on an optical gain region 54, a phase adjusting region 55, and a distributed Bragg reflector (DBR) region 56. The tuning or changing of a selected wavelength is performed by injecting carriers into either or both the phase adjusting region 55 or/and the DBR region 56. The change in the refractive index caused by a plasma effect due to the change in the carrier density is utilized, and the distribution feedback wavelength is tuned by this change in the refractive index or the selected transmission wavelength.
As discussed above, in the prior art tunable optical filter, the refractive index is changed utilizing a plasma effect, due to the carrier injection, to achieve the tuning of the selected wavelength, but when the carrier injection is conducted, optical gain and spontaneous emission light intensity are also changed along with the change in the refractive index. Therefore, in order to both obtain a stable wavelength selectivity and sufficiently suppress crosstalk with a non-selected wavelength, it is necessary to restrict the amount of carrier injection. As a result, the tunable or changeable range of the selected wavelength is necessarily limited.
The change in optical gain is large in a semiconductor optical filter utilizing resonator type optical amplification, and the optical gain is greatly fluctuated by a bias current I.sub.b injected into the DBR region 56, as shown in FIG. 2. In this case, the current injected into the optical gain region 54 is adjusted to compensate for that fluctuation, but at the same time the temperature of the device fluctuates and the refractive index in the device is also changed. Thus, such an adjustment is complicated, and an external electric circuit for conducting the adjustment becomes large in size.
Further, the active layer 51, separate from the waveguide layer 52, is used in the gain region 54, and therefore, the optical coupling design and manufacturing process are sophisticated.